1. Field
Example aspects described herein generally relate to dental restorations, and, in particular, to the use of CAD/CAM dentistry to produce a dental prosthetic.
2. Description of Related Art
Restoring or repairing one or more of a patient's teeth often involves the preparation and attachment of dental prosthetics such as crowns and bridges. Preparation of the dental prosthetic typically is preceded by preparing a tooth for the prosthetic, including removing any damaged, diseased, or decayed areas, removing (and/or resurfacing) hard tissue of the tooth into a shape suitable for a dental prosthetic, and preparing the preparation margin. The preparation margin is the portion of the tooth which will define the interface between the dental prosthetic and the unrestored surface of the hard tooth tissue. For example, when the dental prosthetic will be a crown, the preparation margin may be a ridge located at or near the gingival sulcus, the interface between the tooth and the surrounding gingiva.
Preparation of a dental prosthetic also is preceded by making an impression of the patient's jaw in the area of the restoration site, including the prepared teeth and the surrounding gingiva within the restoration site. In many instances, a physical impression is made. This provides an imprint of the restoration site, which often is made using an intraoral mold, and from which the dental prosthetic is produced. Alternatively, a digital impression can be made from three-dimensional (3D) image data of the restoration site, such as an optical impression taken with a visible-light camera. Because dental prosthetic manufacturing typically relies on a physical model of the restoration site, making a prosthetic from a digital impression requires a computer-aided design/computer-assisted manufacturing (CAD/CAM) system. Regardless of whether an impression is physical or digital, the impression should accurately reflect the physical features of a prepared tooth, particularly the preparation margin, and its surrounding gingiva. An accurate impression can yield a well-fitting dental prosthetic that is secure on the patient's tooth, and that is long-lasting and aesthetically pleasing. An ill-fitting dental prosthetic, on the other hand, can increase the patient's risk of infection or disease, and cause shifting in the prepared tooth and adjacent teeth.
Obtaining an accurate impression is not an insignificant task. A common way to obtain a physical impression of the dental site is to use a fluid mold material that can harden into a solid, such as polyvinyl siloxane (PVS). The mold material is used in combination with an impression tray to hold the fluid. The tray containing the fluid mold material is inserted into the patient's mouth and pressed onto the restoration site. The material then hardens intraorally, creating a permanent impression of the restoration site.
This procedure may be complicated, however, when the patient's soft gingival tissue obscures the preparation margin or otherwise interferes with the mold at or near the preparation margin. As a result of the obstruction, the mold material often cannot accurately record the preparation margin and, in turn, a dental prosthetic produced from the impression may not fit well onto the prepared tooth. Although increased pressure may be applied to the tray during mold hardening to displace the soft tissue, this often is not particularly effective, especially when the margin lies at or below the gingival margin.
There are several existing techniques for exposing the preparation margin when taking an impression. Some techniques rely on displacement of the soft gingival tissue. One example is “packing cord,” a process in which one or more pieces of retraction cord are inserted into the gingival sulcus. The cord forces expansion of the gingival sulcus and displaces gum tissue away from the sulcus, thus creating physical separation between the gingival tissue surrounding the prepared tooth and the tooth itself. However, packing cord is a time consuming process. It also can cause significant pain to the patient and lead to irreversible damage of the gingiva. Another technique for displacing the gingival tissue is the use of a gingival retraction paste, such as Expasyl, a commercial paste manufactured by Kerr Corporation, which can be inserted into the gingival sulcus, where it creates a physical separation between the gingival tissue and the tooth. Because the paste must be removed prior to taking the impression, however, retraction of the gingival tissue is not permanent, and the tissue can rebound to its unretracted position and interfere with the impression. While some pastes include a hemostatic agent to assist with gingival separation after removal of the paste, this only increases the time of the temporary retraction.
Other techniques are directed to removing soft gingival tissue. For instance, electrosurgical devices or soft tissue lasers can be used to remove gingival tissue and expose the preparation margin. Removing gingival tissue, however, may not be adequate to sufficiently expose the margin. Also, tissue removal can be permanent, a consequence that may be adverse to the patient's oral health or cosmetically unappealing.